1. Field of the Invention
The present invention relates to a semiconductor light-emitting device such as a light-emitting diode (LED) or a laser diode (LD), and in particular to a light-emitting device having a semiconductor structure formed of all nitride semiconductor materials.
2. Description of the Related Art
As materials for a light-emitting device such as an LED or LD device which is expected of emitting a light ranging from ultraviolet to red, nitride semiconductor materials (InxAlyGa1−x−yN; 0≦x, 0≦y, x+y≦1) are known, and blue and blue-green LEDs have been put to practical use in, for example, displays or signals.
The light-emitting device such as a blue LED or a blue-green LED formed of nitride semiconductor materials and now actually used has a double-heterostructure. This light-emitting device is fundamentally constructed such that an n-type contact layer consisting of an n-type GaN, an n-type clad layer consisting of an n-type AlGaN, an n-type active layer consisting of an n-type InGaN, a p-type clad layer consisting of a p-type AlGaN and a p-type contact layer consisting of a p-type GaN are superimposed on a substrate made for example of sapphire in the order mentioned. The active layer is doped with a donor impurity such as Si or Ge and/or an acceptor impurity such as Mn or Mg. The light emission wavelength of the LED device can be changed from the ultraviolet region to the red region by varying the content of In in the composition of InGaN of the active layer or by changing the kind of impurities to be doped into the active layer. The LED now put to practical use is an LED having an emission wavelength of 510 nm or less, with its active layer doped with both of donor and acceptor impurities. This LED is has a buffer layer formed of, for example, GaN or AlN between the substrate and the n-type contact layer.
On the other hand, there have been many proposals on a structure of an LD device to date. For example, Unexamined Japanese Patent Application Publication (Kokai) 6-21511 discloses a separation confinement type LD having a structure wherein an active layer consisting of InGaN and having a thickness of not more than 100 angstroms is interposed between an n-type GaN layer and a p-type GaN layer, and the resultant composite is further sandwiched between an n-type AlGaN layer and a p-type AlGaN layer. Both of the AlGaN layers functioning as a light confinement layer.
Through the realization of a double-heterostructure in an LED device as explained above, it has become possible to improve the light-emitting output, thus enabling the LED device to be actually used. However, since an InGaN layer doped with impurities is utilized as an active layer in this LED device, this LED device is accompanied with a drawback that the half band width of emission spectrum can not avoid from becoming wide. For example, a full color display provided with an LED device having a luminescent spectrum of wide half band width will exhibit a whitish luminescent light, thus narrowing the color-reproducing region thereof.
As for LD device, it is theoretically possible to realize a laser oscillation in a double-heterostructure comprising an active layer formed of a non-doped InGaN as described in the above-mentioned Unexamined Japanese Patent Application Publication 6-21511, but the laser oscillation is not yet realized with such a double-heterostructure. It is expected to greatly improve the emission output by making the active layer into a quantum well structure as described in this Publication. However, there are many problems to be solved, such as the preparation of an optical resonance surface or of an optical confinement layer, before the LD device can be actually realized.
In order to realize an LD device, the active layer thereof is required to exhibit a sharp and strong band-to-band emission. Even with an LED device, it is possible to obtain an emission spectrum having a narrow half band width, provided a band-to-band emission can be realized. However, the thickness of the active layer in the conventional LED device is relatively thick, i.e., 0.1 to 0.2 μm, thus the thickness of InGaN layer hetero-epitaxially grown on an AlGaN layer already exceeds over the critical thickness, so that it was impossible with the conventional LED device to realize a strong band-to-band emission, thus failing to realize a laser oscillation. Meanwhile, if an LED device is formed into a quantum well structure by greatly thinning the active layer as shown in the above-mentioned Unexamined Japanese Patent Application Publication 6-21511, it may be possible to obtain a strong band-to-band emission. However, if the thickness of the active layer is thinned, the light confinement may then become insufficient thus making it impossible to realize a laser oscillation.